1. Field of the Invention
The present invention relates generally to the manufacture of plastic containers, and more particularly to blow molded plastic containers having an imbedded oxygen-scavenger.
2. Related Art
In recent years, plastic containers have come into frequent use as replacements for traditional glass containers. Typically these plastic containers are made by a blow molding process such as extrusion blow molding, injection blow molding and stretch blow molding. The process generally involves inflating, with pressurized air, a softened tube of polymer into a mold and then cooling the resulting container. In extrusion blow molding, a hollow polymer tube is extruded. One end of the tube is clamped in a mold, forming the bottom of the container. The tube is then inflated with compressed air through the open end, or through a needle inserted into the tube. In injection blow molding, a core pin is inserted into a mold. Plastic is extruded from the core pin forming a test-tube like shape. The core pin with plastic tube is then placed in a second mold where air is blown through the core pin causing the tube to expand into the mold. Stretch blow molding is similar to injection blow molding, except that the tube is preformed and must be heated before being blown. Blow molding is well known to those skilled in the art.
Blow molding can be used to form containers from a variety of plastics, such as, for example, polyolefins, nylon, and polyesters. Polyolefins include polypropylene, high density polyethylene and low density polyethylene. Polyesters include polyethylene terephthalate (PET).
The use of plastic containers, particularly blow molded plastic containers, offers several advantages over glass containers. For example, plastic containers are generally cheaper to produce; require less energy for production; are generally lighter in weight, thus reducing shipping costs; and are more recyclable. However, the use of plastic containers has several drawbacks. One of the primary uses for plastic containers is in packaging food and beverages, for example juices, applesauce and the like. Many beverages are often placed in containers using a hot fill process. In the hot fill process, the product is added to the container at an elevated temperature, the container is capped and the packaged product allowed to cool. The cooling process results in the formation of a vacuum in the now closed container. While the rigidity of glass containers allows them to easily withstand these vacuum forces, more pliable plastic containers can be subject to distortion under such conditions. One of the initial drawbacks when using plastic as a replacement for glass was in producing a container with sufficient structural integrity to withstand the elevated temperatures and vacuum formed during the hot fill process. This has been accomplished in now well established ways by the use of structural features, for example ribs, that maintain the shape of the container and vacuum panels that flex to assist in relieving the stresses caused by the vacuum.
As uses for plastic containers as a replacement for glass increases, additional challenges are presented. For example, many fruit juices and food products, such as, for example, cranberry juice, grapefruit juice, orange juice, sports drinks, spaghetti sauce, applesauce and the like, are susceptible to oxidation from atmospheric oxygen. In contrast to glass, plastics, and particularly PET, one of the most common plastics used for containers, are generally at least partially oxygen permeable. Thus, upon storage, oxygen from the atmosphere tends to migrate into the container where it can react with the contents. As a result, the shelf-life of these oxidizable contents in plastic containers is reduced relative to the shelf-life when packaged in less permeable glass containers. Several methods have been developed to reduce the oxygen permeability of plastic containers. The most commonly used methods are the use of an oxygen barrier layer or an oxygen scavenger that reacts with oxygen as it diffuses through the container.
Oxygen barrier layers are generally used in two ways. In a first approach, a curable polymer, for example an epoxy-amine composition, is applied as an oxygen barrier over the bottle after it is blow molded. Such layers are disclosed in, for example, U.S. Pat. No. 5,489,455 to Nugent et al. This approach has several disadvantages. For example, the application and curing of the coating adds steps, and therefore time and expense, to the manufacturing process. In a second approach, a container is formed using an oxygen barrier layer as one of several layers in a container. The oxygen barrier layer can be a polymer that is less permeable to oxygen than the base polymer. Examples of plastics that can be used as oxygen barriers include nylon and poly(ethylene co-vinyl alcohol) (EVOH). This layer may be attached to or sandwiched between a structural layer. This approach also adds complexity and cost to the manufacturing process. For example, expensive multiple extruders must be used in the molding process; compatibility, particularly the ability of the oxygen barrier material to bond with the structural resin may present difficulties or require the use of yet another adhesive layer, and the cost of the barrier resin may be high.
In the oxygen scavenging approach, blow molded plastic bottles, for example, PET bottles, have oxygen scavenging components added to the base plastic. Suitable oxygen scavenging components are well-known in the industry. Oxygen scavengers within the PET walls, such as, for example Amosorb® available from BP Amoco Chemicals, react with oxygen diffusing through the package sidewall as well as with oxygen inside the package, both in the packaged product and in the headspace remaining after packaging. See, for example, U.S. Pat. No. 6,083,585.
Many of the conventionally used oxygen scavengers are activated by low levels of moisture. This causes the scavengers to react with oxygen immediately upon the production of an article, such as a preform or a bottle. As this oxygen reacts with the oxygen scavenger, the oxygen scavenging ability of the article declines, even before product is put in the container. Moreover, once the container having the oxygen scavenger is formed, the thin sidewalls allow oxygen to diffuse into the walls at a faster rate where it can react with the oxygen scavenging components. As the oxygen scavenger is consumed, the scavenging ability of the container is reduced. This further decreases the potential shelf-life of the packaged product before packaging and therefore makes storage of the empty bottles problematic.
One solution to this problem requires the use of stretch blow molding techniques. Because preforms are much thicker than bottles, the diffusion of oxygen through the sidewalls of a preform is very slow, relatively small amounts of the oxygen scavenging compound are consumed, and little scavenging ability is lost. Thus, one method of preventing pre-packaging oxygen diffusion is to store the preforms rather than the bottles. The bottles are then blow molded just before they are needed. However, this creates warehousing and scheduling difficulties in the production environment. Furthermore, this is not a viable solution when containers are molded at one facility and shipped to another facility for filling.
Another method of minimizing pre-packaging oxygen diffusion is to make multilayer containers. Multilayer containers can have a central layer that contains the oxygen scavenging compound, and outer layers that “sandwich” the inner layer. The outer layers do not typically contain the oxygen scavenging compound, and serve as barriers to slow the diffusion of oxygen into the inner layer. However, the process of using compressed air to blow multilayer containers forces the oxygen in the compressed air into the non-reactive outer layers, where it can then more readily diffuse into the inner layer. Again, as the container walls become thin after blow molding, the rate of oxygen diffusion into the walls increases and the shelf-life is compromised. In addition, as with the use of multilayered bottles incorporating oxygen barrier resins, the use of multiple layers presents similar problems in terms of machine complexity.
There is thus a continuing need for improved methods of preparing and storing blow molded plastic containers for packaging oxygen sensitive products where the containers include containing oxygen scavenging components or oxygen barrier materials that overcome the shortcomings of conventional solutions. There is a particular need for achieving these goals in a manner that allows the use of monolayer resins in bottle manufacturing.